Conventional x-ray imaging is based on the absorption of x rays as they pass through different parts of a patient's body. Depending on the absorption in a particular tissue such as muscle or lung, a different intensity of x rays will pass through and exit the body. During conventional x-ray imaging, the exiting or transmitted x rays are recorded with a detection device, such as an x-ray film or other image receptor, and provide a two dimensional projection image of the tissue within the patient's body. While these images can be very useful, when there is one structure in front or behind another, their images are superimposed in a single projection image and it is impossible to know which is in front, and each may obstruct the visibility of the other.
While also based on the variable absorption of x rays, Computed Tomography (CT) imaging provides a different form of imaging known as cross-sectional imaging. CT imaging, also known as Computerized Axial Tomography (CAT) scanning, has been developed and used for many years to generate images of cross-sectional planes or slices of anatomy. Each image is generated by a computer synthesis of x-ray transmission data obtained in many different directions in a given plane. Because CT scans reveal organs, bone, blood vessels, and soft tissues, including lung, muscles, and tumors, with great clarity and detail, CT systems are particularly useful as a diagnostic or therapeutic guidance tool for medical purposes. CT systems have also been known to be useful in industrial, security, and other systems where imaging data are to be obtained.
A CT system typically has a circular opening and includes an x-ray source and a detector array. A motorized table is commonly used to move a subject to be examined, such as an object, a patient, or a region of interest thereof, up or down and in or out of the circular opening. The x-ray source and the detector array are mounted on opposite sides of a rotating gantry. As the patient passes through the system, the x-ray source rotates around the inside of the circular opening. The x-ray source produces x-rays that pass through the patient and impinge on the detector array, which may be arc-shaped and also revolving. This process is also known as scanning.
In known third generation CT systems, the x-ray source, comprising an x-ray tube, provides x-rays emanating from a point commonly referred to as a “focal spot”. The x-ray beam emanating from the focal spot to the array of detectors resembles the shape of a fan and therefore is sometimes referred to as a “fan beam”. The narrow, fan-shaped beam of x rays is used to irradiate a section or slice of the patient's body. The thickness of the “fan beam” may be as small as 0.5 millimeter or as large as 10–20 millimeters. A typical scanning process usually involves many rotations, each generating a different slice. Thus, the scanning process could involve dozens or hundreds of rotations of the x-ray source around the patient in coordination with the motorized table through the circular opening.
The x-ray source is coupled to the detector array in a manner that the focal spot of the x-ray tube and the detector array are on one plane. The x-ray source and the detector array rotate together about an axis of rotation, such as an axis through the patient, perpendicular to the plane. For each position of the rotating gantry, the detector array records x rays exiting the section of the patient's body being irradiated as a projection, also known as a view or an x-ray profile. Many different views are collected during one complete rotation, typically a 360 degrees rotation. A single rotation takes about 1 second. During each rotation, the detectors may record about 1,000 views (x-ray profiles). The x-ray projection data collected/measured/sampled are then sent to a computer for reconstructing all of the individual views into a cross-sectional image (slice) of the internal organs and tissues for each complete rotation. Multiple computers are typically used to control the entire CT system. Despite the discrete nature of the sampled data, a number of known reconstruction algorithms are able to convert the collected data into high quality images of the slice. In this scanning mode, a three dimensional image can be made by producing images of slices, one at a time. The thickness of the slice and the spacing between slices are adjustable.
Most modern CT imaging systems are capable of performing “spiral”, also called “helical”, scanning as well as scanning in the more conventional “axial” mode as described above. Spiral CT systems are well known in the art. An exemplary teaching can be found in U.S. Pat. No. 5,966,422, “MULTIPLE SOURCE CT SCANNER,” issued on Oct. 12, 1999 to Dafni et al. and assigned to Picker Medical Systems, Ltd. of Haifa, Israel.
Briefly, the term “spiral” comes from the shape of the path, relative to the object, taken by the x-ray beam during scanning. The motorized (examination) table advances at a constant rate through the scanning gantry while the x-ray source rotates continuously around the patient, tracing a spiral path relative to the patient. This spiral path gathers continuous x-ray profile data without gaps. The pitch of a spiral scan, or helix, is defined as how far the patient is translated during one rotation divided by the thickness of the fan beam. In a typical procedure, the pitch ranges from about 1 to 2. A single rotation takes approximately 0.5 to 1 second.
Some CT imaging systems, also called multi-detector CT or multi-row CT systems, are capable of imaging multiple slices simultaneously, allowing relatively larger volumes of anatomy to be imaged in relatively less time. In such a system, a number of planes or slices are sampled simultaneously via multiple rows of detectors. Since data for several slices can be obtained in one scan, total scanning time is greatly reduced. Exemplary teachings can be found in U.S. Pat. No. 6,047,040, “DETECTOR SIGNAL INTEGRATION IN VOLUMETRIC CT SCANNER DETECTOR ARRAYS,” issued to Hu et al. on Apr. 4, 2000; and U.S. Pat. No. 6,137,857, “SCALABLE DETECTOR FOR COMPUTED TOMOGRAPH SYSTEM,” issued to Hoffman et al. on Oct. 24, 2000 and assigned to General Electric Company of Milwaukee, Wis., USA.
However, these multi-detector or multi-row CT systems still require more than one revolution during image scanning to produce data for a thick volume. A logical extension to the multi-slice or multi-detector scanning mode is called a cone beam CT. The goal is to enable the reconstruction of an entire three-dimensional (3D) object using a single rotation. Unfortunately, this imaging geometry suffers from several known drawbacks such as image artifacts, sometimes referred to as cone beam artifacts, and image reconstruction errors. Briefly, the divergence of the x-ray beam in the direction of the axis of rotation causes the reconstruction problem to be ill-posed. Indeed, even in multi-row CT systems, as the angle spanned by the multiple rows increases, so do the image reconstruction problems caused by the image artifacts. As such, the system will increasingly suffer from cone beam artifacts. Additionally, if one tries to reduce these artifacts by using more accurate reconstruction algorithms, image reconstruction can be computationally intensive and slow in cone beam CT systems.